Logger on_message trigger (#374)

* Docs

* Review

* Note added, example updated

* Fix RST code syntax. Expand note.


Co-authored-by: Otto Winter <otto@otto-winter.com>

components/logger.rst

@@ -159,6 +159,35 @@ Configuration options:
 -  **tag** (*Optional*, string): The tag (seen in front of the message in the logs) to print the message
    with. Defaults to ``main``.
 
+Logger Automation
+-----------------
+
+.. _logger-on_message:
+
+``on_message``
+**************
+
+This automation will be triggered when a new message is added to the log.
+In :ref:`lambdas <config-lambda>` you can get the message, log level and tag from the trigger
+using ``message`` (``const char *``), ``level`` (``int``) and ``tag`` (``const char *``).
+
+.. code-block:: yaml
+
+    logger:
+      # ...
+      on_message:
+        level: ERROR
+        then:
+          - mqtt.publish:
+              topic: some/topic
+              payload: !lambda |-
+                return "Triggered on_message with level " + std::to_string(level) + ", tag " + tag + " and message " + message;
+
+.. note::
+
+    Logging will not work in the ``on_message`` trigger. You can't use the :doc:`logger.log <logger-log_action>` action  
+    and the ``ESP_LOGx`` logging macros in this automation.
+
 See Also
 --------
 

guides/automations.rst

@@ -321,6 +321,7 @@ All Triggers
 - :ref:`binary_sensor.on_click <binary_sensor-on_click>` / :ref:`binary_sensor.on_double_click <binary_sensor-on_double_click>` /
   :ref:`binary_sensor.on_multi_click <binary_sensor-on_multi_click>`
 - :ref:`esphome.on_boot <esphome-on_boot>` / :ref:`esphome.on_shutdown <esphome-on_shutdown>` / :ref:`esphome.on_loop <esphome-on_loop>`
+- :ref:`logger.on_message <logger-on_message>`
 - :ref:`time.on_time <time-on_time>`
 - :ref:`mqtt.on_message <mqtt-on_message>` / :ref:`mqtt.on_json_message <mqtt-on_json_message>`
 - :ref:`pn532.on_tag <pn532-on_tag>` / :ref:`rdm6300.on_tag <rdm6300-on_tag>`